An electric vehicle such as a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and a battery-only electric vehicle (BEV) has an electric motor and a traction battery. The motor is interposed between the battery and a drive shaft of the vehicle, wherein the motor is coupled to the driveline of the vehicle. The motor may be controlled to use energy from the battery to contribute positive wheel torque to the wheels of the vehicle in order to propel the vehicle.
Conversely, the motor may be controlled to contribute negative wheel torque to the wheels for vehicle braking. During a process referred to as regenerative braking, charging the battery during vehicle deceleration collects the kinetic energy stored in the braking vehicle. During regenerative braking, required wheel braking torque is allocated between friction brakes and the motor, which acts as a generator to charge the battery.
The capability of a typical traction battery to collect regenerative braking energy becomes limited as the temperature of the battery falls and is especially noticeable below freezing. A method for heating the battery in order to overcome this reduction in collection capability involves cycling energy into and out of the battery. However, at relatively low temperatures relatively little energy can be put back into the battery. In this case, the discharge current provides all of the battery heating.